1. Field of the Invention
The present invention relates generally to the field of direct current-to-alternating current (DC/AC) inverters, and more particularly to control schemes for a multilevel inverter.
2. Description of the Related Art
A DC/AC inverter changes DC power into AC power. In a PWM (pulse width modulation) controlled inverter, DC voltage is “chopped” into pulses. The mean value of the pulses follows a sinusoidal waveform. Depending on the power rating of the inverter, the switching frequency can range from a few kHz to a few tens of kHz. There are two main issues amongst many others in design of such an inverter-high switching power loss and high electric magnetic interference (EMI).
One of the alternatives to PWM controlled inverter schemes is to use a multilevel voltage source inverter. A multilevel inverter is a practical approach when an output transformer is required at the load side for safety reasons. Standard products with power ratings of around 5kW are available in the market, such as the SW series from Trace Engineering, 5916 195th St. N.E. Arlington, Wash. USA 98223. The advantages of using a multilevel inverter over a PWM controlled inverter include very good surge capability, high efficiency, good voltage and frequency regulation, and low total harmonic distortion (THD).
FIG. 1 shows a typical configuration of a multilevel inverter 100. A single DC power source 102 provides power to several low-frequency H-bridge circuits 104 and output transformers 106. The number of low-frequency H-bridge circuits 104 and output transformers 106 is often determined by specific application requirements.
Each H-bridge circuit 104 may generate a zero voltage or a positive/negative step voltage at its output. A series connection 108 of the output transformers 106 provides a multilevel voltage waveform to an AC load (not shown).
FIG. 2 shows an output voltage waveform of the multilevel inverter 100, which can be used to generate an approximation of a desired output voltage waveform (see FIG. 4). The voltage “steps” shown in FIG. 2 are associated with energizing the individual output transformers 106 in series connection 108 (FIG. 1) in various and different ways, as explained below. With respect to each individual output transformer 106, the transformer output voltage is proportional to the number of turns of the transformer windings. For a specific application, the number of turns of a winding is fixed.
There are many possible transformer turn ratio configurations. For example, assume the turn ratio between the three transformer output windings (W1, W2, W3) is 9:3:1. The corresponding voltage ratio will also be 9:3:1. With this voltage ratio, assuming the waveform is quarter symmetrical, a possible composition for a quarter cycle may be designed as,
Step No.12345678910111213Output1(3 − 1)3(3 + 1)(9 − 3 − 1)(9 − 3)(9 − 3 + 1)(9 − 1)9(9 + 1)(9 + 3 − 1)(9 + 3)(9 + 3 + 1)
At step 1, W3 is energized to positive step voltage, while W1 and W2 are not energized. At step 2, W2 is energized to positive, W3 is energized to negative, while W1 is not energized. Similarly, 13 voltage steps are created in the first half cycle with equalized voltage increase at each step. A maximum of 27 (including zero) voltage steps may be created within each cycle (60 Hz). The number of steps per cycle required is based on the level of DC input voltage and the percentage of output load.
Once the voltage steps are fixed, the times at which the step voltages occur (also referred as switching times) are specified. There are many related art approaches to calculating the switching time. Harmonics elimination and harmonics minimization schemes are two of the optimal approaches.
Detailed optimal calculation of switching time based on the mathematical theory of harmonics elimination and harmonics minimization schemes is complicated and time consuming. See J. Chiasson, etc., “Real-Time Computer Control of a Multilevel Converter using the Mathematical Theory of Resultants”, Electrimacs, August 2002. Consequently, practical real time applications of such related art schemes generally use look-up tables containing pre-calculated switching times. At run time, look-up tables are routinely selected to provide desired output voltage with optimized THD.
Because the related art schemes of multilevel inverter control typically use look-up tables at run time, such related art schemes are unresponsive to real time changes involving multilevel inverters. In addition, due to the fact that the control equations are non-linear and numerous, real time application of such equations is not generally practicable. Accordingly, a need exists in the art for a method and system that can provide multilevel inverter control in a fashion that is less computationally intensive than the control used in related art systems. In addition, a need exists in the art for a method and system that can provide multilevel inverter control in a fashion that is responsive to real time changes involving multilevel inverters.